A wavelength tunable photo-thermal common-path interferometer (PCI) system has been created to measure low level absorption of optical materials in the infrared wavelength range from 2.3 mu m to 3.6 mu m. The focus of this paper is to discuss the PCI technique and the challenges of measurement calibration. The PCI system is used to characterize optical grade ZnSe and spinel. Measured wavelength-dependent absorption coefficients for these materials are presented and the potential of the data to alter or augment existing material absorption models is discussed.

Measurements of the bulk optical absorption coefficient are important for the design of imaging systems operating with high temperature windows because the emissivity of the window is related to the bulk absorption coefficient. Photothermal common-path interferometry (PCI) offers an all-optical technique to measure the absorption of optical window materials. When combined with laser heating, PCI can be used to measure the temperaturedependent, bulk absorption coefficient in window materials over a wide range of temperatures. In this paper, we demonstrate the PCI technique at elevated temperatures achieved using laser heating.

Antireflection (AR) coatings are essential to the performance of optical systems; without them, surface reflections increase significantly at steep angles and become detrimental to the functionality. AR coatings apply to a wide range of applications from solar cells and laser optics to optical windows. Many times, operational conditions include high temperatures and steep angles of incidence (AOIs). The implementation of AR coatings is extremely challenging in these conditions. Nanoporous coatings made from high-temperature-tolerant materials offer a solution to this problem. The careful selection of materials is needed to prevent delamination when exposed to high temperatures, and an optimal optical design is needed to lower surface reflections at both the normal incidence and steep AOIs. This paper presents nanoporous silicon dioxide and hafnium dioxide coatings deposited on a sapphire substrate using oblique angle deposition by electron beam evaporation, a highly accurate deposition technique for thin films. Developed coatings were tested in a controlled temperature environment and demonstrated thermal stability at temperatures up to 800°C. Additional testing at room temperature demonstrated the reduction of power reflections near optimal for AOIs up to 70° for a design wavelength of 1550 nm. These findings are promising to help extend the operation of technology at extreme temperatures and steep angles.

Optical window characterization is performed with a CO2 laser heating the material to understand the optical effects, thermal effects, and temperature dependence of the index of refraction. Distortions in the optical window caused by operating in challenging aerothermal environments can impact an imager’s performance. Uneven heating of the window will induce a temperature gradient, which when coupled with the temperature dependence of the refractive index, causes a flat sapphire window to act as an imperfect lens. Experimental capability allows multiple sensors and diagnostic equipment to collect synchronized data. A long-wave infrared (LWIR) camera images the sample’s front and back surfaces to measure temperatures and temperature gradients. A transmitted laser probe beam is captured simultaneously by a visible imager and wavefront sensor. The visible imager captures how a point source appears observed through the window. A transmitted wavefront is reconstructed from the wavefront sensor. The reconstructed wavefront includes effects from both dn/dT and mechanical deformation of the window. Using the reconstructed wavefront and imager optics in Zemax, the point spread function (PSF) of the imager looking through the heated window is generated and compared with the experimentally measured PSF.

Sapphire should be highly transparent for photon energies less than the band gap, but residual, weak absorption and scattering losses in the near infrared occur as a result of extrinsic and intrinsic defects. Lattice disorder, impurities, and point defects have all been implicated as being the origin of loss phenomena but very little experimental evidence exists to quantitatively establish the relationships that might exist between these defects and optical loss. In this study, three synthetic, c-axis sapphire samples manufactured under similar conditions were characterized using UV-VIS spectroscopy, photothermal common-path interferometry, and positron annihilation lifetime spectroscopy. Model-based interpretation of optical measurements indicated that vacancy-type defects were partially responsible for absorption loss from the ultraviolet to the near-infrared and that the population densities differed among the samples. Positron annihilation lifetime spectroscopy measurements also indicated a higher concentration of cationic vacancy defects near the sample surface which correlates with a higher surface optical loss. This work establishes the use of positron annihilation techniques as a characterization tool for optical materials that could be useful for investigating the origin of weak surface absorption in the transparent region of sapphire.

A combination of experimental lidar results and shape-dependent scatter amplitude matrix calculations are used to explore the utility of polarimetric lidars for aerosol characterization. Solutions are developed for the induced polarization vector response of skewed spheroidal particles and the scatter is then computed using an improved anomalous diffraction approximation method. Experimental data was collected on biological simulant, chemical simulant, and interferent aerosol clouds using a 1047 nm micropulse lidar designed to measure the simultaneous depolarization using a linearly polarized source. Depolarization signatures obtained during testing show a clear difference between wet and dry biological simulant aerosols, wet chemical simulant releases, and some interferents. Combining these measurements with the shape-dependent model calculations help us understand the unique polarimetric signatures that may be exploited for aerosol characterization using stand-off lidar techniques.

Durable window materials with minimal optical loss are important for future high-energy laser (HEL) systems that will operate at or near the megawatt level. Sapphire is recognized as a promising HEL window candidate due to its outstanding optical transparency and mechanical properties. However, its weak scattering characteristics and absorption levels near the 1-mu m wavelength region need to be lowered further for future HEL applications. In our study, the weak absorption and scattering of sapphire samples provided by three vendors were measured. Ultraviolet-visible spectroscopy measurements were made in the wavelength range 190 to 600 nm and several absorption bands due to vacancies and impurities were detected. The bulk absorption of several samples at wavelengths of 355, 532, and 1064 nm were measured using photothermal common-path interferometry and the absorption coefficient values obtained were in the range of 10(-5 )to 10(-2) cm(-1) and increased as the wavelength decreased. An empirical weak absorption tail model was used to fit the measured data. In addition, scattering measurements on all samples were made at 405, 532, 633, 1064, and 1550 nm using an instrument developed to assess the bidirectional scatterance probability distribution function. The total integrated scatterance was in the range of 10(-4) to 10(-2) and increased for all samples as the wavelength decreased. Surface roughness was found to contribute insignificantly to the scattering loss, while bulk defects along with subsurface damage have major impact. A simple single-scatter model was developed and applied to the measured bulk scattering data. The model suggests that impurity particles, porosity, and other density variations exist with a range of sizes that contribute to scattering. Overall, the measurements indicate that both weak absorption and scattering losses are strongly related to defect structures such as lattice disorder and impurities that were introduced during crystal growth or postgrowth processing. Understanding these defects and their contributions to optical loss can lead to improved manufacturing and processing methods. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)

A 1989 SPIE proceedings paper described the first version of an optical properties computer code developed at the Applied Physics Laboratory. The emphasis was on durable infrared window materials. In the thirty years since the beginning of this code development a lot of progress has been made concerning the physics based models used to represent the complex index of refraction and the number of materials characterized. A key feature is providing the frequency and temperature dependence of optical properties. Room temperature data is available for a wide range of materials, but temperature dependent data is much harder to find. The status of the code development along with example outputs will be reported.

Temperature-dependent diffuse reflectance measurements on Cr-doped alpha-alumina monoliths have been performed using supercontinuum-laser illumination and CO2-laser heating. These measurements have been interpreted using an extended Kubelka-Munk (K-M) model describing diffuse-light propagation in highly scattering and fluorescent media to assess the temperature dependence of fluorescence quantum efficiency. Analysis of experimental results has provided a qualitative understanding of the temperature-dependent conditions for model applicability and also suggests methods for using supercontinuum-laser diffuse reflectance spectroscopy for detection of unknown fluorescent dopants. (C) 2019 Optical Society of America

Supercontinuum-laser illumination in conjunction with CO2-laser heating has been implemented to measure the near to short-wave infrared (970-1660 nm) diffuse reflectance of plasma-sprayed Nd2Zr2O7 as a function of temperature. Owing to the broadband nature of this experimental technique, the diffuse reflectance of plasma-sprayed Nd2Zr2O7 has been measured at many wavelengths and has been shown to decrease with increasing temperature. A physics-based model for diffuse reflectance predicated on the crystal/electronic band structure of highly scattering semiconductor materials has been constructed to interpret the results of these measurements. Baseline materials characterization has also been performed to assist in the development of crystal/electronic band structure-optical property relationships that could be useful for the design of next-generation environmental barrier coatings. This characterization has included ambient and non-ambient x-ray diffraction as well as room-temperature, integrating-sphere diffuse reflectance spectroscopy.

This study presents results for the high-temperature (up to 1550 K) optical properties of polycrystalline Cr-doped a-alumina materials. Diffuse reflectance spectra in the wavelength range of 510-840 nm are presented as a function of temperature to illustrate changes to the optical behavior of these materials including a previously unreported thermally activated splitting of the U-band absorption ((4)A(2) -> T-4(2)) in octahedrally coordinated Cr3+. Measurements were made using a unique laser-based approach for high-temperature solid-state spectroscopy, involving front-side supercontinuum laser illumination and back-side CO2 laser heating. This approach required development of samples that could withstand related thermal stresses, and measurements were made on plasma-sprayed, Cr-doped alpha-alumina monoliths. Measured spectra are interpreted, in part, using published optical spectra for ruby; agreement between results here with those obtained using more traditional methods serves to validate the measurement methods used for this work. (C) 2017 Optical Society of America

This paper presents the first, to our knowledge, direct measurement of aerosol produced by an aluminized solid rocket propellant (SRP) fire on the ground. Such fires produce aluminum oxide particles small enough to loft high into the atmosphere and disperse over a wide area. These results can be applied to spacecraft launchpad accidents that expose spacecraft to such fires; during these fires, there is concern that some of the plutonium from the spacecraft power system will be carried with the aerosols. Accident-related lofting of this material would be the net result of many contributing processes that are currently being evaluated. To resolve the complexity of fire processes, a self-consistent model of the ground-level and upper-level parts of the plume was determined by merging ground-level optical measurements of the fire with lidar measurements of the aerosol plume at height during a series of SRP fire tests that simulated propellant fire accident scenarios. On the basis of the measurements and model results, the Johns Hopkins University Applied Physics Laboratory (JHU/APL) team was able to estimate the amount of aluminum oxide (alumina) lofted into the atmosphere above the fire. The quantification of this ratio is critical for a complete understanding of accident scenarios, because contaminants are transported through the plume. This paper provides an estimate for the mass of alumina lofted into the air.

Temperature dependent transmittance derived line width and line shift measurements are conducted on polycrystalline 1% and 6% Nd doped YAG in the 293 K-473 K range. A single crystal temperature dependent line width model is adapted for polycrystalline YAG. Comparison between line width measurement techniques was conducted, and the transmittance method is preferred for ground state line width measurements. Polycrystalline YAG material is found to have broader intrinsic line width than single crystal material. Polycrystalline YAG material should provide mode locking advantages of shorter minimum pulse length than single crystal YAG.

his article discusses a series of optical techniques for assessing the performance of heated IR windows. The assessment considers the complete thermo-mechanical behavior as well as the boresight error and image quality of the sensor that looks through the window. The entire discussion serves as a vehicle for emphasizing the role that fundamental research plays in the everyday mission of APL. We demonstrate that research into the fundamental properties of matter not only provides insight into the behavior of the material, but often leads to the creation of secondary tools or procedures that in turn can be used to assess system performance.

Applications involving space based instrumentation and aerodynamically heated surfaces often require knowledge of the bi-directional reflectance distribution function (BRDF) of an exposed surface at high temperature. Addressing this need, the Johns Hopkins University Applied Physics Laboratory (JHU/APL) developed a BRDF facility that features a multiple-port vacuum chamber, multiple laser sources covering the spectral range from the longwave infrared to the ultraviolet, imaging pyrometry and laser heated samples. Laser heating eliminates stray light that would otherwise be seen from a furnace and requires minimal sample support structure, allowing low thermal conduction loss to be obtained, which is especially important at high temperatures. The goal is to measure the BRDF of ceramic-coated surfaces at temperatures in excess of 1000 degrees C in a low background environment. Most ceramic samples are near blackbody in the longwave infrared, thus pyrometry using a LWIR camera can be very effective and accurate.

In order to determine true radiometric quantities in intense fires a three dimensional (3D) understanding of the fire radiometric properties is desirable, e.g., for estimating peak fire temperatures. Imaging pyrometry with a single infrared camera view can provide only two dimensional path-averaged radiometric information. Multiple camera views, however, can form the basis for determining 3D radiometric information such as radiance, emissivity, and temperature. Analytically the fire can be divided into sub-volumes in which radiometric properties are assumed roughly constant. Using geometric and thermal equilibrium relationships between the fire sub-volumes, together with LWIR camera imagery acquired at multiple carefully defined camera views, radiometric properties of each sub-volume can be estimated. In this work, initial proof-of-principle results were obtained by applying this analysis to sets of LWIR camera imagery acquired during intense (2500 – 3000 K) fires. We present 3D radiance and temperature maps of the fires obtained using this novel approach.

The Earth albedo, or the ratio of the upwelling to down-welling radiative flux at the surface, consolidates both wavelength-and angle-dependent information, often oversimplifying surface properties. Accurate characterization of this parameter is important for many Johns Hopkins University Applied Physics Laboratory (APL) programs. Within the albedo value is the bidirectional reflectance distribution function (BRDF), which describes the angle dependency of reflectance and emission relative to observation angle, solar angles, and wavelength. As a first step toward improving land characterization of Earth and assisting with removal of land radiative effects to reveal pristine target signatures, we developed a semiempirical BRDF model for leaves. Leaves are easy to acquire, easy to integrate into an existing facility, and highly seasonally and regionally dependent. Typical leaf reflectances used in modeling cover a wavelength of similar to 500-2000 nm. We extended this database from 250 to 10,000 nm to examine the radiative impact of surface albedo. In this article, we present the model and compare it with quantitative observations for red maple, dogwood, and white oak leaves. Implementing the default parameterization of a maple leaf in the Moderate Resolution Radiative Transfer (MODTRAN) radiative transfer code results in a retrieved radiance error of 1-3% relative to our improved BRDF model. This error is outside the required absolute accuracy for environmental and atmospheric models used at APL and highlights the need to use our new parameterization for environmental studies.

The optical properties of polycrystalline and single crystal Nd:YAG materials are reported. Materials include undoped, 1%, 1.5%, 2%, 4%, 6%, and 10% at. Nd doped polycrystalline YAG and undoped and 1% at. Nd doped single crystal YAG. The motivation of this paper is to determine the ideal material type and doping percentage gain media for use in a high energy 0.946 μm Nd:YAG laser. Room temperature transmittance measurements for all samples are performed between the midinfrared and ultraviolet edges of absorption. From this calibrated transmittance data, an extinction coefficient and extinction cross section are calculated across the entire spectra. We determine that, for the doping percentages measured, the extinction cross section is independent of doping level. Our measured extinction cross sections are consistent with previously reported values. Temperature dependent transmittance measurements are conducted on 1% and 6% polycrystalline Nd:YAG material. A classical oscillator model for the diode pump band absorption coefficient is presented. A comparison of strengths and weaknesses of single crystal and polycrystalline is presented. This comparison reveals areas of parasitic absorption in polycrystalline YAG.

Elastic backscatter LIght Detection And Ranging (LIDAR) is a promising approach for stand-off detection of biological aerosol clouds. Comprehensive models that explain the scattering behavior from the aerosol cloud are needed to understand and predict the scattering signatures of biological aerosols under varying atmospheric conditions and against different aerosol backgrounds. Elastic signatures are dependent on many parameters of the aerosol cloud, with two major components being the size distribution and refractive index of the aerosols. The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has been in a unique position to measure the size distributions of released biological simulant clouds using a wide assortment of aerosol characterization systems that are available on the commercial market. In conjunction with the size distribution measurements, JHU/APL has also been making a dedicated effort to properly measure the refractive indices of the released materials using a thin-film absorption technique and laboratory characterization of the released materials. Intimate knowledge of the size distributions and refractive indices of the biological aerosols provides JHU/APL with powerful tools to build elastic scattering models, with the purpose of understanding, and ultimately, predicting the active signatures of biological clouds.

The (heuristic) Kubelka-Munk theory of diffuse reflectance and transmittance of a film on a substrate, which is widely used because it gives simple analytic results, is compared to the rigorous radiative transfer model of Chandrasekhar. The rigorous model has to be numerically solved, thus is less intuitive. The Kubelka-Munk theory uses an absorption coefficient and scatter coefficient as inputs, similar to the rigorous model of Chandrasekhar. The relationship between these two sets of coefficients is addressed. It is shown that the Kubelka-Munk theory is remarkably accurate if one uses the proper albedo parameter.